After all that.....ST70

Got my re-cap project complete and had fun....but no change in the hum problem with inputs unplugged or when unplugging one side! New caps on PCB, new quad cap, bias caps and upgraded the diode. So, I have a sweet little amp that is still not "perfect" and although that bugs me I learned a lot. In looking at the forums, seems these have a grounding or ground loop problem that affects many amps, and also seems the sources are hard to trace. I have new input jacks that are grounded to the chassis, and I see that some recommend isolating them...and also replaced the two-prong cord with a grounded three-prong. Who knows? Other recap issues I ran into: pulled about a quarter inch of tracer off the board and one of the sockets when pulling out an old cap, ended up jumping the tear with a discarded lead from one of the old caps, a pretty "duct tape" solution but seems to run OK. Also UNsoldering the old stuff is really really tough....old brittle solid wire, usually cut way too short by whomever was building the kit, wrapped around the connection like three times and crimped and soldered....really hard to get off to replace components. But got it done.

Thanks to all for the help. IA

Disconnect the ground wire on that power cord from the chassis and tape it off. This eliminated the hum on my unit.

Thanks guys....the plate on the cage says "Dynakit" stereo 70 so I guess this is a kit and the two resistors off the input jack to ground are probably already on the board? I will also take off the 3-prong - these days adding a ground seems like the smart thing to do but from what I read causes more problems with this amp than it solves. I am also going to try and isolate the input jacks from the chassis...thanks again.

Here are links to photos of my PCB and also photo of my new inputs. I hope I do not win the new ST70 "abortion" award by placing 1/4" phone jacks as inputs, completely bypassing the old RCAs and the mono/stereo switch. I do music and recording and almost all my stuff is phone jacks, even input/outputs on the new recording interfaces, so for convenience and flexibility that's what I did....hot and ground of the phone jacks are wired directly to the sockets on the board, but both are also currently grounded to the chassis.

Believe it or not, the board looks like this after two alcohol and toothbrush treatments so it's hard to see. If you do see the two resistors are NOT on the board, I assume I need to 1) add a 470k resistor from the tip of each phone jack to ground (chassis??); and 2) isolate the jacks totally from the chassis so there is no direct ground. Are these resistors available at my local Radio Shack? Will also use a "cheater" plug to test grounded plug v.s. non polarized. You guys are great. Thanks for help. Bob, do you live on the Cape? We live in Iowa but go to Wellfleet each year. IA.

http://showcase.netins.net/web/wildiowaphotos/PCboard.JPG
http://showcase.netins.net/web/wildiowaphotos/Frontshot.JPG

David and Bob,

Do you also recommend that folks disable their pesky ground fault outlets and use AC equipment in the bathtub? This is dangerous advice that you are casually tossing around.

Dan

Hi Bob,

I appreciate that you are willing to listen to other opinions - I value that trait very highly.

There are indeed thousands of ungrounded ST-70s floating around, but on this forum we are dealing with folks building or rebuilding amps today. They are seeking advice and many of them have little training in electricity. They may not be able to visualize what happens if one of those wires accidentally touches an ungrounded chassis. I have seen pictures of some very scary wiring jobs posted on the web. My view is that forum readers should be informed on what is considered safe practice today and not what was considered acceptable 50 years ago.

Best,
Dan

Since I have already disabled and bypassed the existing RCA jacks and the M/S switch, leaving them in for appearance only, I am going to add a 470k resistor from the tip of the new input jacks directly to chassis ground. Please tell me if this should go to the grounding socket on the board instead, or if the chassis is OK!!! Hope that will address the impedence issue there.

Bob you must live near Sturbridge Village at the 84/90 split. My wife was from Milford MA and have a bro in law in Shrewsbury. Also two daughters in NYC so drive right up 84 to MA Turnpike. This summer I will drop this pesky thing off in person since I will be within a few miles of you and have you do things right! Thanks for help...IA

Postscript: I have two JBL EON speakers we use in the band and one has a hum...we lift the ground with a cheater and everything is cool. Very strange stuff these ground loops.

wildiowa wrote:Since I have already disabled and bypassed the existing RCA jacks and the M/S switch, leaving them in for appearance only, I am going to add a 470k resistor from the tip of the new input jacks directly to chassis ground. Please tell me if this should go to the grounding socket on the board instead, or if the chassis is OK!!! Hope that will address the impedence issue there.

Probably the easiest would be to solder the 470K resistors directly between the tip and ground tabs on the 1/4" jacks. I'm sure you are probably aware that Switchcraft, among others, makes nylon body jacks which would be insulated from the chassis.

Best Regards,
Charlie

Update on ST70 rebuild....replaced two EL34's on one side and inserted missing resistor between tip and ground on new input jacks and things have improved dramatically. I ran into a local dude who claims he knows tubes and he gave me a couple 100k resistors for the inputs rather than the recommended 470k, said that would be allright. Is this OK? Hum seems to have disappeared and amp sounds very nice. Still have not isolated new inputs from chassis ground, need nylon jacks or fibre washers but when it was fired up yesterday things seem to be OK. This has been a fun journey thanks to all who provided advice and parts etc.

Are you still using that stock pc board?? if so you have to get rid of that. i had boards that looked like that on mk3's, actually not as bad. i replaced the boards with glass epoxy boards designed just like stock. it resolved tons of noise and hum problems. its said that those old boards absorb moisture and i believe it. also the blackened ares can be slightly even more conductive.

I ran with the stock design for many years and then in jan 2012 upgraded to roys 6sn7 drivers which really opened up the sound even further.

at the very least rebuild the stock circuit on a glass epoxy board often sold on ebay. i used new precision matched parts.